Slow Translational Diffusion Research Articles

NMR and NQR study of ClF5 in its solid phases

The 19F NMR line shape and relaxation have been studied in solid ClF5 from the melting point down to liquid nitrogen temperature. The corresponding results, completed by relaxation data concerning 35Cl NMR and NQR, have yielded information about phase transitions and molecular motions. In particular, a new solid–solid transition was found at 117 °K. 35Cl wide-line and 19F high resolution NMR spectra were obtained in the high temperature phase SI, which is shown to be a plastic phase where molecular reorientations (τϑ=8×10−12 sec) are nearly as fast as in the liquid. In addition to the reorientation there is a much slower translational diffusion, the main characteristics of which are derived from 19F T1 and T1ρ measurements using Torrey’s theory. In the low temperature phases SII and SIII, the 19F T1 is due to rotations of the molecules by jumps around their C4 axis (τr=10−8 sec at 110 °K). In solid II the NQR relaxation time T1Q is due to another mechanism which is assumed to be jumps of the C4 axis between two equilibrium positions. The dipolar energy relaxation time TD was also measured; between 123 and 153 °K, the Cl–F dipolar coupling modulated by the chlorine relaxation appears to be the main TD mechanism, for which a theory is described.